When an electron in an emitter (e.g., an atom or a molecule) is excited, the electron can relax by emitting light through various transition processes. These processes include, for example, multipolar process in which the orbital angular momentum of the electron changes by more than one, multiphoton process involving emission of two or more photons upon each transition, and spin-flip transitions where the electronic spin is flipped. Having access to these transition processes can benefit a wide range of technological areas.
For example, access to multipolar transitions can yield data on the electronic structure of atoms, molecules, and condensed matter by spectroscopic means. Moreover, access to multipolar transitions can also determine many electronic transitions within a short period time, thereby paving the way for single-shot spectroscopy.
In another example, having access to spin-flip transitions such as the singlet-triplet transition can be beneficial not only for spectroscopic applications, but also for quenching of organic molecules, which are often used in organic light sources. For example, in organic-dye lasers, a persistent challenge is that the population of excited electrons usually rapidly fills a triplet state. Because the singlet-triplet transition is slow, the electrons can remain in the triplet state for a long time. Increasing the population in the triplet state can accordingly decrease the population in the singlet state for lasing transitions. A high population of triplet states can also increase the lasing photon absorption and the number of molecules that are permanently lost through photobleaching, thereby limiting the lasing efficiency. Quenching of triplet states can reduce the population in triplet states, thereby increasing the lasing efficiency.
In yet another example, two-photon spontaneous emission processes can be used to construct broadband light sources. For example, a fast two-photon emitter can be used to turn a monochromatic beam of light into a broadband beam of light.
However, one common challenge to utilize the above mentioned transition processes is that these processes are usually too slow to be observable. The slow emission, without being bound by any particular theory or mode of operation, can be attributed to the long wavelength of the emitted light compared to the size of the emitter. Typically, the wavelength of light is about 1000 times to about 5000 times larger than the size of an atom. Currently, accessing the large set of extremely slow light emission processes remains a challenging technological problem.